1. Field of the Invention
The present invention relates to long cycle life solid-state galvanic cells comprising cation conducting solid polymer electrolytes and intercalation materials for both the positive and negative electrodes. Reduction of morphological phase changes at the electrode/electrolyte interfaces during electrochemical cycling contributes to the long cycle life and stable operation of the solid-state cell configurations of this invention.
2. Description of the Prior Art
Secondary cells utilizing essentially pure lithium as negative electrodes with lithium ion conducting non-aqueous electrolytes generally exhibit less than Faradaic cycling efficiency. Lithium electrodes are prone to undergo surface morphological changes during electrochemical cycling which lower the overall coulombic efficiency of the cell. The reduction in coulombic efficiency represents an irreversible loss in lithium capacity after each cell cycle. During cell charging, electrodeposited lithium reacts with the non-aqueous electrolyte to form an insulating film at the lithium electrode/electrolyte interface. This electrochemically deposited lithium film is non-uniform and dendritic areas develop which become electrically isolated from the lithium negative active material. During subsequent discharge, lithium particles become susceptible to mechanical removal from the electrode without contributing to the overall Faradaic charge capacity of the electrode. Lithium particles lost in this manner are generally unavailable for further cell cycling. This type of irreversible lithium loss due to morphological changes at the lithium electrode/non-aqueous electrolyte interface occurs when unit activity lithium is deposited during electrode charge. Dendrite formation is also a problem in solid-state cells utilizing lithium ion conducting solid polymer electrolytes with lithium electrodes.
The importance of intercalation compounds in solid state chemistry is known. See, e.g., M. B. Armand, "Intercalation Electrodes", Materials for Advanced Batteries, D. W. Murphy and J. Broadhead, eds., Nato Conference Series VI, p.145 (1979). Intercalation compounds undergo topochemical reactions involving the insertion of a guest into the intercalation compound host lattice structure with minimal structural changes by topotactic electron/ion transfer. Intercalation reactions are generally completely reversible at ambient temperatures and pressures, and therefore utilization of intercalation compounds in secondary cells is very promising.
Lithiated rutiles have been utilized as electrodes in rechargeable electrochemical cells. The topochemical lithiation of rutile related structures in non-aqueous lithium electrochemical cells is taught in D. W. Murphy et al, "Topochemical Reactions of Rutile Related Structures with Lithium", Mat. Res. Bull. 13: 1395 (1978). This article relates to the use of transition metal chalcogenides, oxides and oxyhalides as host structures suitable for use as cathodes in room temperature batteries utilizing lithium as the guest. Rutile related metal dioxides, in particular, exhibit a range of important parameters for lithium incorporation which suggest their suitability for high energy density battery applications, such as the range of size and vacancy for lithium, diffusion pathway, electronic conductivity, and crystallographic distortion.
One researcher suggests that intercalation of lithium ions may be achieved by reaction of the host lattice with a lithium/ammonia solution. R. Schollhorn, "Reversible Topotactic Redox Reactions of Solids by Electron/Ion Transfer", Angew. Chem. Int. Ed. Engl. 19: 983 (1980). This article also teaches that much experimental work has been conducted with Li/TiS.sub.2 cells having a solid lithium anode and TiS.sub.2 layered dichalcogenide cathode. The role of ternary phases in lithium anodes and cathodes comprising metallic halide, oxide and chalcogenide intercalation compounds is elucidated in M. S. Whittingham, "The Role of Ternary Phases in Cathode Reactions", J. Electrochem. Soc., 123: 315 (1976).
Cells have also been proposed having two intercalation electrodes, each intercalation electrode having a different lithium activity. The validity of this system was tested in lithium organic electrolyte cells comprising Li.sub.x WO.sub.2 and Li.sub.y TiS.sub.2 reversible electrodes and LiClO.sub.4 -PC electrolyte. M. Lazzari and B. Scrosati, "A Cyclable Lithium Organic Electrolyte Cell Based on Two Intercalation Electrodes", J. Electrochem. Soc., 127: 773 (1980).
Galvanic cells utilizing iron-doped beta-alumina cathodes are operable at closer to ambient temperatures than conventional sodium-sulfur beta-alumina systems. Cells comprising sodium negative electrodes, beta-alumina ceramic electrolyte, and sintered beta-alumina positive electrodes in which some of the aluminum sites were replaced by iron in the beta-alumina structure operated at about 120.degree. C. were shown to be electrochemically regenerative. "Galvanic Cells Containing Cathodes of Iron-Doped Beta-Alumina", J. H. Kennedy and A. F. Sammells, J. Electrochem. Soc. 121: 1 (1974).
A variety of solid polymer electrolytes has been proposed for use in solid-state rechargeable electrochemical cells. In general, the conductivity of solid polymer electrolytes, such as poly(ethylene oxide) (PEO), is too low for ambient temperature operation, but the use of thin films and elevated temperatures of about 40.degree. to about 120.degree. C. increases conductivity and enhances performance for electrochemical cell applications. The feasibility of thin film polymer electrolytes for high energy density, high power density cells was demonstrated for solid-state cell configurations with lithium anodes, poly(ethylene oxide) complexed with LiClO.sub.4 or LiCF.sub.3 SO.sub.3 supporting electrolyte, and TiS.sub.2 and V.sub.6 O.sub.13 cathodes. M. Gauthier et al, "Assessment of Polymer-Electrolyte Batteries for EV and Ambient Temperature Applications", J. Electrochem. Soc. 132: 1333 (1985). This article teaches that cells utilizing LiCF.sub.3 SO.sub.3 supporting electrolyte demonstrated poor utilization of the TiS.sub.2 electrode, and required high operating temperatures and reduced current density compared to cells utilizing LiClO.sub.4 supporting electrolyte.
Properties of ionic complexes of poly(ethylene oxide) (PEO) relating to their suitability for use as battery electrolytes have been investigated. Poly-ethers, specifically poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) give adducts with selected alkali metal salts which are suitable for use as thin film solid-state electrolytes. M. B. Armand et al, "Poly-Ethers as Solid Electrolytes", Fast Ion Transport in Solids, Vashishta, Mundy, Shenoy, eds. (1979). This article also teaches that different polymer conductivities observed at different temperatures relate to cation size, salt concentration and crystallinity. Results of studies concerning the temperature dependence of dc conductivity of poly(ethylene oxide) complexes with sodium iodide and thiocyanates of sodium, potassium and ammonium are reported in P. V. Wright, "Electrical Conductivity in Ionic Complexes of Poly(ethylene oxide)", Br. Polym. J. 7: 319 (1975). Measurements of transference numbers for ion conducting polymers are reported in R. Dupon et al, "Transference Number Measurements for the Polymer Electrolyte Poly(ethylene oxide) NaSCN", J. Electrochem. Soc., 128: 715 (1981). The morphology of polymer crystalline structures and morphological implications for ionic conductivity are investigated in C. C. Lee and P. V. Wright, "Morphology and Ionic Conductivity of Complexes of Sodium Iodide and Sodium Thiocyanate with Poly(ethylene oxide)", Polymer, 23: 681 (1982) and D. R. Payne and P. V. Wright, "Morphology and Ionic Conductivity of Some Lithium Ion complexes with Poly(ethylene oxide)", Polymer, 23: 690 (1982).
The suitability of ionic complexes of other solid-state polymers for battery electrolytes is taught in the prior art. A variety of metal salt complexes of polyphosphazenes demonstrate good conductivity at room temperature, which suggests their suitability for room temperature thin film battery electrolyte applications. P. M. Blonsky et al, "Polyphophazene Solid Electrolytes", J.Am.Chem. Soc. 106: 6854 (1984). Conductive poly(vinyl acetate) complexes are taught in M. C. Wintersgill et al, "Electrically Conducting Poly(Vinyl Acetate)", J.Electrochem. Soc. 131: 2208 (1984). Preparation of conductive solid electrolyte complexes of poly(ethylene succinate) is described in R. Dupon et al, "Ion Transport in the Polymer Electrolytes Formed Between Poly(ethylene succinate) and Lithium Tetrafluoroborate", J.Electrochem. Soc. 131: 586 (1984). Rechargeable organic batteries utilizing thin films of polyacetylene reversibly electrochemically doped to give a series of organic metals as negative and/or positive active material and liquid non-aqueous electrolytes are taught in D. MacInnes, Jr., et al, "Organic Batteries: Reversible n- and p- Type Electrochemical Doping of Polyacetylene, (CH).sub.x ", J. Chem. Soc., Chem. Commun., p.317 (1981).